1. Field of the Invention
This invention relates to a key assignor having a keyboard multiplexer for electronic musical instruments.
2. Description of the Prior Art
In apparatus having a large number of key switches, such as an electronic organ, a large amount of wiring is required for directly connecting the key switches to desired circuits so that on-off information of the former may be supplied to the latter. To avoid this, there has usually been employed a time division multiplex system in which the key switches are scanned on a time shared basis to obtain and apply the on-off information in the form of a TDM (Time Division Multiplex) or PCM (Pulse Code Modulation) signal to a key code memory. With this system, however, since the on-off state of the key switches is checked on the time shared basis, information of the key switches in the off state is also sent to the key code memory. This requires one scanning period for sending required information and a clock of a very high frequency, for instance, several hundred kilohertzes, for providing a rapid response to key depression and release. Further, the response time lags due to the relationship of the moments of key depression and release to one scanning period: for example, when a depressed key switch is released immediately after being scanned, the response time lags about one scanning period. An improvement that has been made on the abovesaid method is to temporarily stop scanning of an output line for a predetermined period of time upon key code generation by key depression and to rapidly effect the aforesaid scanning while no key code signal is generated. Also, with this method, however, whether depressed or not, the key switches are all scanned through at a high speed, so that the scanning is still time-consuming. Moreover, circuit integration imposes a limitation on high-speed scanning.
In U.S. Pat. No. 3,981,217 there is described another key assignor for use in an electronic musical instrument which is capable of detecting changes in key switches by comparing the previous on/off state of the key switches with previous on/off states thereof. However, the present invention distinguishes over that patent in the provision of a second memory circuit and in the method of feeding back processed depressed key information.
In the aforementioned referenced patent, a signal selected by the priority circuit directly controls the key code generator. In the present invention, a signal selected by the priority circuit is temporarily stored in the second memory circuit, and by the stored signal, the key code generator is activated.
With the feedback method of the referenced patent, a key code assigned is read from a key code memory circuit, decoded, and then stored in a key code signal generator, thereafter being fed back to a change detector. In contrast thereto, in the present invention, a depressed key signal is transferred from the second memory to a third memory and then fed back to the inhibit circuit. Accordingly, in the present invention, the depressed key signal is applied from the third memory directly to the inhibit circuit.